1. Field of the Invention
The present invention relates to a legged mobile robot.
2. Description of the Related Art
There is known a legged mobile robot equipped with an upper body and legs, each leg including a hip joint connecting the upper body and a thigh link, a knee joint connecting the thigh link and a crus link, and an ankle joint connecting the crus link and a foot, which moves by driving the legs.
For example, Japanese Patent Laid-Open No. 2004-298997 discloses that the knee joint connecting the thigh link and the crus link has a knee Y axis (a pitch axis), and an output power from an electric motor arranged in the thigh link is transmitted to the knee Y axis via a belt and a reducer. A predetermined point of the crus link moves relative to the thigh link taking the knee Y axis as the center of rotation, generating a circular trajectory.
However, in the legged mobile robot disclosed in the above-mentioned Japanese laid-open patent, when the knee joint is almost fully stretched out, that is, when a joint angle (bending angle) of the knee joint stretches to the vicinity of 0 degree, the joint angle hardly varies even when driving power is transmitted to the crus link, so that the robot falls into a situation where drive controllability is impaired. Therefore, the knee joint can only be extended to the joint angle having good drive controllability.
Therefore, even when the robot is in the upright standing state, there is a necessity of bending the knee joint to a certain degree. Therefore, large torque must be acted on the knee joint, so that a problem that the driving source must be made larger and heavier arises. Further, a problem that a frequency of bending and stretching the hip joint and ankle joints are increased also arises. Still Further, the knee joint of a supporting leg cannot be made in a state where it is almost stretched out, so that the knee joint projects forward, and becomes susceptible to interfere with the knee of a free leg. In accordance thereto, a problem that the freedom of degree of walking gait is small, and that it is inferior in moving stability (balance) arises.